This invention relates to activated carbon material that has improved iodine retention properties and, to a method of treating said material to improve its iodine retention characteristics, in particular at elevated temperatures. This product is particularly useful as an efficient filter or trap for extracting radioactive iodine components from the gaseous effluents of nuclear facilities.
Activated carbon, in the physical form of granular charcoal beds, has been extensively used for the extraction of elemental iodine and iodine compounds from the gaseous streams of nuclear facilities. See Nuclear Safety, Vol. 9, No. 5, pp 373-382, 1968. Carbon beds are used as traps for fission product iodine both in nuclear reactor confinement systems and in nuclear fuel processing facilities. Untreated activated carbon will remove a substantial portion of the iodine content of airborne radioactivity when the iodine isotopes are present in elemental form. Depending upon the compound form, the temperature, and the humidity of the system, iodine removal efficiencies ranging from less than 10% to about 99% are possible with untreated material. However, present standards require that iodine removal from the effluents of these confinement systems approach 100%.
In attempts to improve the iodine retention characteristics of activated carbon, various chemical treatments have been used. One well known treatment is to iodize the activated carbon using KI or I.sub.2. The best results were obtained with coconut base carbon, but a uniform product was not always obtained. For example, samples of iodized carbon (nominally 5% KI-I.sub.2 on 1500-m.sup.2 /g coconut base carbon) tested at 180.degree. C showed a variation in iodine penetration rate ranging from 0.023% to 12.8%. The results of tests of coal and petroleum base carbons that were iodized by conventional techniques do not even approach the lower range of the coconut base carbons in iodine retention. Another treatment technique comprises impregnating activated charcoal with certain water-soluble secondary and tertiary amines. Triethylenediamine (TEDA) impregnation, in particular, enhances the affinity of charcoal for the compound methyl iodide. This latter technique is disclosed in U.S. Pat. No. 3,453,807, issued to Roland Taylor on July 8, 1969. Unfortunately, in the most effective percentage range of TEDA, the amine addition substantially reduces the ignition temperature of the impregnated charcoal, rendering it unsuitable for use in the nuclear facility environment wherein ignition temperatures for filter media below about 300.degree. C cannot be tolerated or are unacceptable under current safety standards.
In view of these limitations, it is, therefore, an object of the present invention to provide an improved activated carbon material having improved iodine retention characteristics. It is another object of the invention to provide a method for improving the iodine retention properties of activated carbon without reducing the ignition temperature of the carbon below acceptable limits. It is a further object of the invention to provide an activated carbon material having improved radioiodine retention properties at elevated temperatures.